1. Field of the Invention
The present invention relates generally to a sensor system for detecting a mix ratio of a fluid mixture passing through a cylindrical pipe. More specifically, the invention relates to a sensor system for detecting mixing ratio of mixture of petroleum fuel and alcohol which is to be introduced into an engine.
2. Description of the Background Art
In recent years, mixtures of petroleum fuel and alcohol have been used as fuel for driving engines and so forth in order to decrease dependency upon petroleum. In a case where such a mixed fuel is used, it is required that the mixing ratio of petroleum fuel to alcohol is detected for controlling a fuel injection amount and a spark ignition timing on the basis of the detected mixing ratio so as to always obtain optimum output if the mixing ratio varies.
One conventional sensor system for detecting the mixing ratio of the mixture of petroleum fuel and alcohol is shown in FIG. 1. This sensor system has a cylindrical container 50 made of metal, which is connected to an earth cable 51 to serve as an external electrode. Connectors 52 and 53 are mounted on both end portions of the cylindrical container 50 to define therein a fuel passage 54. An internal electrode 55 is arranged within the fuel passage 54 so that both end portions thereof are supported on support members 56 and 57 made of an insulating material. A bolt 59 is inserted into a through opening formed in the cylindrical container 50 via a bush 58. The bolt 59 is formed with a screw portion at the end thereof, and the screw portion engages the internal electrode 55. In addition, the bolt 59 is connected to a lead 60.
After fuel hoses are connected to the connectors 52 and 53, when the fuel F, which is a mixture of petroleum fuel and alcohol, passes through the cylindrical container 50, a condenser 65 is formed in which the fuel F serves as a dielectric material between the cylindrical container (external electrode) 50 and the internal electrode 55 as shown by an equivalent circuit of FIG. 2.
Generally, electrostatic capacity C [the fuel F] of the condenser 65 can be expressed by the following formula. ##EQU1## A: Area of Electrode [cm.sup.2 ] t: Distance between Electrodes [cm]
.epsilon.: Dielectric Constant of Dielectric Material [F/cm] PA0 .epsilon..sub.O : Dielectric Constant in Vacuum (=8,854.times.10.sup.-4) [F/cm] PA0 .epsilon./.epsilon..sub.O : Dielectric Constant of Dielectric Material
Therefore, if A and t are constant, electrostatic capacity C [F] varies in proportion to the dielectric constant .epsilon./.epsilon..sub.O of dielectric material, so the dielectric constant of dielectric material, i.e. fuel F, can be obtained by detecting electrostatic capacity C [F]. For example, dielectric constants of gasoline and methanol are about 1.95 and 32.4, respectively. Since dielectric constant of a mixed fuel varies in accordance with its mixing ratio, the mixing ratio of the mixed fuel F can be obtained by detecting its electrostatic capacity C [F].
FIG. 4 is an example of variation of electrostatic capacity C relative to mixing ratio of a mixture of petroleum fuel and methanol. From this graph, it is understood that the mixing ratio of the mixed fuel F can be obtained by detecting electrostatic capacity C.
This electrostatic capacity C is input to, e.g. an oscillating circuit 66 as shown in FIG. 3. This oscillating circuit 66 outputs an oscillating frequency f determined on the basis of circuit constants, such as resistances R.sub.1, R.sub.2 and R.sub.3, and of electrostatic capacity C, so that variation of the electrostatic capacity, and thereby variation of the mixing ratio, is detected via variation of oscillating frequency f.
However, in such conventional sensor systems, since the electrical connecting portion is closed from the outside of the cylindrical container only by means of the bolt 59, the bolt 59 tends to be pushed out to cause leakage of fuel when fuel pressure increases.
In order to eliminate the aforementioned disadvantage, the inventors of the present invention have previously disclosed an improved fluid detecting sensor system as shown in FIGS. 5(a) and 5(b) in Japanese Patent First (unexamined) Publication (Tokkai Hei.) No. 1-196557. In this sensor system, a cylindrical container 1 made of metal is connected to a lead 2 to serve as an external electrode. The cylindrical container 1 has an open end which serves as an inlet port for introducing fuel F into the cylindrical container 1 and which also serves as a connector. An internal electrode 3 is arranged within the cylindrical container 1. The supply-side end portion of the internal electrode 3 is supported on a support member 4 which is similar to that of conventional sensor systems. If the internal electrode 3 has relatively high rigidity, the support member 4 is not required. The other end portion of the internal electrode 3 is supported on a plug-type support member 5 by which one end of the cylindrical container 1 is closed. In addition, the cylindrical container 1 is in fluid communication with a fuel discharge pipe 6 which projects at a location upstream of the plug-type support member 5 in an essentially radial direction of the cylindrical container 1 and which also serves as a connector.
Connection between the cylindrical container 1 and the fuel discharge piper 6 is performed by, for example, brazing. After rust, stains, and so forth are removed from the inner and outer surfaces, nickeling and so forth are performed.
The plug-type support member 5 has a small-diameter portion 5A on the opposite side to the supply side within the cylindrical container 1. The cylindrical container 1 is formed with a small-diameter supporting portion 1A for receiving therein the small-diameter portion 5A of the plug-type support member 5. An O-ring 7 is arranged between the facing surfaces of the small-diameter portion 1A and the support member 5. In addition, the internal electrode 3 passes through the plug-type support member 5 to project from the cylindrical container 1 to be secured to the end portion of the cylindrical container 1 by means of a nut 10 via a insulating bush 9 made of Teflon or the like which has a small dielectric constant and a small dielectric loss. The projecting end portion of the internal electrode 3 is connected to an output terminal 8.
After fuel hoses are connected to the cylindrical container 1 and the fuel discharge pipe 6, the mixed fuel is introduced into the cylindrical container 1 to be discharged from the fuel discharge pipe 6, so that the mixing ratio of the mixture of gasoline and alcohol can be measured in a manner similar to the aforementioned manner. Generally, pressure of this mixed fuel increases to about 10 kg/cm.sup.2 to thrust the plug-type support member 5 toward the small-diameter portion 1A. This thrusting force is received by the small-diameter portion 1A via the O-ring 7, so that the O-ring 7 deforms to improve sealing performance between the support member 5 and the cylindrical container 1 so as to prevent leakage of fuel F.
However, in this type of sensor system, manufacturing accuracy of the contact surface of the cylindrical container 1, which comes into contact with the O-ring 7, becomes difficult due to thermal distortion when brazing, so that sealing performance of the O-ring 7 becomes insufficient. That is, since the contact surface, particularly the inner peripheral surface, of the cylindrical container 1 is formed by inserting a tool, such as a drill, into the cylindrical container 1 from the left in FIG. 5(b), manufacturing accuracy of the contact surface of the cylindrical container 1 can not be optimally formed when the cylindrical container 1 distorts due to brazing of the fuel discharge pipe 6 to the cylindrical container 1.